Cell-like structures of the honeycomb type in thermofusible material have been produced industrially for about twenty years. The most used polymer is polypropylene (PP) because of its lightness, of its low cost, of the ease of its application, of its excellent resistance to humidity and to most chemicals. Its recycling is easy. Honeycombs in polycarbonate (PC) and in polyetherimide (PEI) occupy niche markets, and there exist productions, for the time being marginal productions, in polyvinylchloride (PVC), in polyethylene terephthalate (PET) and in cellulose triacetate (CTA).
The main commercial outlet for these products is the sandwich panel, for which they make up the core. The upper and lower skins of the sandwich are thin and dense materials, for which the elastic tensile modulus is generally high. The sandwich structure with a cell-like core of the honeycomb type (i.e. for which the axis of the cells is perpendicular to the plane of the sheet, and designated in this way although the section of the cell is not always hexagonal) provides a rigidity/weight ratio and a compressive resistance/weight ratio without any counterparts. This is why building, automotive, furniture, aerospace, railway transport industries as well as ship-building increasingly resort to this technique.
Polypropylene honeycomb structures with very wide cells (up to 50 mm) are also used naked, or with a non-woven heat-adhered on their surface in order to prevent filling-in of the cells, in the fields of civil engineering and landscaping: reduced backfills, rainwater management.
The honeycomb core is generally made by extruding tubes (WO 94/25258) or elements with a rectangular section (WO 87/00119) having hexagonal cells. The tubes receive by co-extrusion an external cladding of a polymer with a lower melting point which will be used as an adhesive. These tubes or these beams are then cut to the intended length and stacked in order to form blocks. In the case of tubes, by having them pass in a long hot air oven, it is possible to melt the adhesive and after cooling, a consolidated block is obtained, easy to cut with a band saw in order to obtain plates with the desired length. The beams are then welded against each other with a hot blade (mirror weld).
These methods have a certain number of significant drawbacks: the use of a hot melt adhesive with a low melting point limits the thermomechanical resistance of the cell structure to a level which is clearly inferior than that of the constitutive polymer of said structure. Further, in order to withstand the pressure exerted on its flanks by the jaws which transport it through the oven, even if hot air softens it, the tubular honeycomb block should have minimum density. It is 80 kg/m3 with polypropylene in the case of cells of small dimensions (typically a section of 8 mm) for the sandwich panel application. Now, for the large majority of the applications, one hardly needs more than 55 kg/m3. This technical problem therefore prevents optimization of the weight and of the cost of the finished product.
Another consequence is that the cutting of the block into slices with a hot wire, which is more aesthetical than cutting with a band saw, can only be accomplished at a maximum rate of 10 cm per minute and with a risk of thermal degradation in the case of a polypropylene block with a density of 80 kg/m3, and it is therefore not used. It becomes advantageous in the case of lighter honeycombs; thus 30 cm per minute may be attained with a polystyrene block with a density of 55 kg/m3. On the other hand, by using stacking containers and jaws it is not possible to obtain blocks with a length of greater than 3 meters which complicates and in certain cases excludes the manufacturing of panels of large dimensions. There is also the problem posed by the extrusion screws, which are designed for a given type of polymer which prevents the manufacturer from proposing honeycombs in very different materials. Finally, it is impossible to transport a compact semi-finished product and to assemble it as a honeycomb with lightweight industrial means as close as possible to large users. As a conventional trailer truck can hardly take away more than 60 m3 of honeycomb, the transport costs quickly become prohibitive.
There exist methods for continuously and directly manufacturing cell structures to the desired thickness. WO-A-9841388 describes the extrusion, by means of a die with several slots, of parallel sheets in a thermofusible material with production of compartments between the sheets. By alternating depressurization and fluid-filling in one compartment out of two, a cell structure is made. This method is more economical than the previous ones only for high plate thickness. Now, the latter is limited by the complexity of the technique to about thirty millimeters. Further, the density of the obtained product is higher.
WO-A-0032382 describes a honeycomb structure made by rotary thermoforming in vacuo. Two thermoformed areas form half-cells and are separated by a planar area being used as a folding line of the sheet along the width. Folding is carried out like an accordion so as to bring the half-cells closer together which, once they are juxtaposed, form a honeycomb structure. The ends of the cells are covered with planar areas. This honeycomb core manufacturing step, for maintaining the shape of the structure, requires that it be immediately followed by a step for adhering the skins forming the outer faces of the panel.
In other words, although the connection of the walls of the half-cells is contemplated in WO-A-0032382, this method requires that the panel be made completely and not only the sole honeycomb core. Further it does not allow modulation of the thickness, which remains very limited because of the thermoforming on a cylinder.